Several dementias, most importantly Alzheimer's disease (AD), are characterized by the formation of intracellular aggregates consisting of the microtubule-associated protein tau, termed neurofibrillary tangles (NFT). The importance of this biochemical abnormality for the clinical syndrome of dementia is illustrated by essentially three facts: (I) there is a close correlation between the state of dementia and the extent and density of NFT in various parts of the cortex [e.g., Bancher C. et al. (1993) Neurosci. Lett. 162, 179-182)]; (ii) individual neurons containing NFT in the cell body and/or the neurites are morphologically degenerating, i.e., lose synaptic connections and eventually die [Braak E. et al. (1994) Acta Neuropathol. 87, 554-567; Callahan L. M. et al., (1995) Neurobiol. Aging 16, 311-314]; (iii) a certain density of NFT in various otherwise unrelated dementias is always associated with dementia, without exception.
The tau protein contained in NFT is severely hyperphosphorylated [Goedert M. et al. (1995) Neurobiol. Aging 16, 325-334; Hasegawa M. et al. (1996) FEBS Lett. 384, 25-30]. This abnormal phosphorylation renders the protein incompetent to retain its original function, i.e., stabilization of the microtubule cytoskeleton, which is of fundamental importance for the integrity of a neuron [Iqbal K. et al. (1994) FEBS Lett. 349, 104-108; Garver T. D. et al. (1996) J. Neurosci. Res. 44, 12-20]. This explains the paucity of intact microtubules in AD brains. Phosphorylation alone is responsible for this effect, as dephosphorylation restores the abilities of tau.
Because of a relationship between tau phosphorylation, cytoskeletal destabilization, synaptic loss and neuronal degeneration, and ultimately dementia, it would be therapeutically desirable to have pharmaceutical means to interfere with the pathological process of tau hyperphosphorylation.
The characteristics of hyperphosphorylated tau in NFT suggest that the protein kinase ERK2 is responsible for the pathological tau modification in AD [Drewes G. et al. (1990) EMBO J. 11, 2131-2138; Roder H. M. et al. (1993) Biochem. Biophys. Res. Commun. 193, 639-647]. ERK2 may exist in an abnormally activated state in AD [Roder H. M. et al. (1995) J. Neurochem. 64, 2203-2212). Inhibition of ERK2 has therefore been suggested as a point of interference to prevent tau hyperphosphorylation, and ultimately to stop NFT formation in neurons.
AD-like tau hyperphosphorylation can be induced in several cellular models (including brain slices), converting tau into a phosphorylation state indistinguishable from tau phosphorylated by ERK2 in vitro. The most convincing cellular models involve PP2A inhibition [Sautier, P. E. et al., Neurodegeneration 3, 53-60 (1994); Harris K. A. et al., Ann. Neurol. 13, 77-87 (1993)].
However, compounds which inhibit ERK2 and thereby prevent AD-like tau hyperphosphorylation in biological model systems, have previously not been disclosed. Such compounds can be expected to affect processes of neurofibrillary degeneration, tied to tau hyperphosphorylation, in a beneficial manner.
The protein kinases of the ERK family, often termed MAP-kinases, have also been implicated in a variety of important cellular regulation events outside the CNS, such as growth, differentiation and inflammation [e.g., Sale E. M. et al., EMBO J. 14, 674-684 (1995); Pages G. et al., Proc. Natl. Acad. Sci. USA 90, 8319-8323 (1993); Cowley S. et al., Cell 77, 841-852 (1994)]. Consequently, aberrant ERK activation has been implicated in several diseases characterized by loss of growth and differentiation control. In some tumors constitutive ERK activation is associated with cellular transformation due to dominant (activating) mutations in signal transduction proteins or viral proteins interfering with ERK inactivators [Sontag E. et al., Cell 75, 887-897 (1993); Leevers S. J. and Marshall C. J., EMBO J. 11, 569-574 (1992); Gallego G. et al., Proc. Natl. Acad. USA 89, 7355-7359 (1992); Gupta S. K. et al., J. Biol. Chem. 267, 7987-7990 (1992)].
The use of the disclosed kinase inhibitors for cancer is also indicated by their ability to inhibit cdc2 kinase. The role of cdc2 and homologous (cdks) kinases in cell cycle control is very well appreciated [Norbury C., and Nurse P., Annu. Rev. Biochem. 61, 441-470 (1992)]. Regulation of these enzymes is essential for both commitment to cell cycle from the resting state (START), and ordered transition through several phases of the cell cycle. The need for regulation is reflected in the existence of numerous positive and negative regulatory features of cdks, such as cyclin subunits, inhibiting (Thr) and activating (Tyr) phosphorylations, and endogenous peptide inhibitors.
Because of this central role of cdks in control of cell cycle and proliferation, they are considered as attractive drug targets for cancer therapies [e.g., Filguera de Azevedo W. et al., Proc. Natl. Acad. Sci. USA 93, 2735-2740 (1996)].